A microwave container has, for example, as disclosed in Patent Literatures 1 and 2 described below, a flange part outwardly extending from an opening, and a rib-shaped heat-seal part is formed over the entire periphery of the upper surface of the flange part. The heat-seal part serves a part for heat seal of a film-shaped lid member for sealing the opening of the container. In a portion of the heat-seal part, a vapor-discharging part for carrying out so-called vapor discharge is formed. The vapor-discharging part is formed of a V-shaped part consisting of an opening pattern widening toward the outside. The vapor-discharging part configured as described above allows a stress to be concentrated on an inner sharpened tip of the V-shaped portion when the vapor generated in the container reaches a predetermined pressure, and therefore breaks the heat seal of the lid member.
However, both the vapor-discharging part and the remaining part of the heat-seal part are equal in heat seal width. Thus, the heat seal is increased in strength when their widths are equally extended, causing an insufficient vapor discharge from the vapor-discharging part. In contrast, when their widths are equally narrowed, the heat-seal part other than the vapor-discharging part inevitably causes an inappropriate vapor discharge.
Furthermore, the vapor-discharging part as configured as described above inevitably has its limitations as follows: In the case of controlling vapor to break the heat seal of the lid member without fail when the vapor reaches a predetermined pressure, there is a possible way to set a heat seal width of the heat-seal part to a predetermined value. In this case, however, such a way should be carried out in a narrow space.